Channel decoders related to base band signal processing of WCDMA and CDMA2000 terminal receivers require sufficiently accurate reliability information on channel bits. The reliability information on the channel bits can be supplied by base band detectors, such as Rakes or equalizers. In general, the reliability information is given to a channel decoder in the form of soft outputs, i.e. by scaling channel bit estimates (“raw bits”) according to their reliability. Given that a hard channel bit decision b(k)=±1, then c(k)=Qb(k) is fed to the channel decoder, where c(k) is a soft channel bit decision at a time instant k, and Q is a scaling factor. A large Q≧0 reflects high reliability of the decision, and vice versa. The value of Q should somehow depend on the signal to noise ratio of the detector output.
In addition to channel decoding, reliability information either in the form of soft outputs or packet signal-to-interference ratio (SIR) is required to perform hybrid-ARQ packet combining of retransmitted data packets in a High Speed Downlink Packet Access (HSDPA) system. Misleading reliability information on a failed packet may deteriorate all subsequent retransmissions of the same packet. In HSDPA systems, a terminal receiver also has to transmit a channel quality indicator (CQI) to the base station for scheduling and selecting an optimal modulation scheme and channel coding rate. CQI can be based on SIR estimate.
A Rake detector performs maximal-ratio combining (MRC), which directly produces a usable soft output. A common pilot channel based SIR estimator may be used for helping hybrid-ARQ packet combining to take into account possibly different SIR levels during different packet retransmissions. The SIR estimator is decoupled to the output of a code correlator bank of a Rake receiver.
Typically, a linear chip equalizer output is decoupled to a code correlator and then to a channel decoder as such, without any additional scaling. However, the equalizer output and the output of the code correlator decoupled to the equalizer output may not carry sufficiently good reliability information on the channel bits. This can be observed especially in channels with a small number of multipaths (e.g. PedA in WCDMA and most of the channels in CDMA2000 systems).
A conventional common pilot channel based SIR estimator can be used at the output of a chip-equalizer correlator chain. The SIR estimator can be similar to that of the Rake but now a single-path channel can be assumed. A conventional SIR estimator can be implemented, for example, by using equation (1):
                              SIR          =                                                                                                                                                            1                        K                                            ⁢                                                                        ∑                                                      k                            =                            0                                                                                K                            -                            1                                                                          ⁢                                                  y                          ⁡                                                      (                            k                            )                                                                                                                                                    2                                                                                                                    K                                                  K                          -                          1                                                                                                                                                                        (                                                                                                            1                              K                                                        ⁢                                                                                          ∑                                                                  k                                  =                                  0                                                                                                  K                                  -                                  1                                                                                            ⁢                                                                                                                                                                      y                                    ⁡                                                                          (                                      k                                      )                                                                                                                                                                        2                                                                                                              -                                                                                                                                                                                    1                                  K                                                                ⁢                                                                                                      ∑                                                                          k                                      =                                      0                                                                                                              K                                      -                                      1                                                                                                        ⁢                                                                      y                                    ⁡                                                                          (                                      k                                      )                                                                                                                                                                                                                          2                                                                          )                                                                                                        ·                                                SF                                      hs                    -                    dsch                                                                    SF                  cpich                                                      ⁢                                          P                                  hs                  -                  dsch                                                            P                cpich                                                    ,                            (        1        )            where SIR is a signal to interference ratio, y(k) is a despread common pilot symbol with the effect of pilot symbol (1+j) removed, SFhs-dsch is a spreading factor for HS-DSCH (High Speed-Downlink Shared Channel), SFcpich is a spreading factor for CPICH (Common Pilot Channel), Phs-dsch is the power of HS-DSCH channel, and Pcpich is the power of CPICH channel.
The numerator of equation (1) is thus a square of the estimate of the pilot symbol amplitude at the ouput of the chip-equalizer correlator chain, and thus it is an estimate of the received common pilot power. A denominator estimates the interference power by subtracting the pilot power estimate from a total power estimate. The SIR estimate is then scaled according to a data channel's spreading factor and power (relative to the common pilot channel).
However, the conventional SIR estimator suffers from estimation noise and from the fact that in order to simplify the SIR estimator, an SIR estimate may be updated e.g. only once per slot. A need exists for an improved and simpler process for an estimator.